The more prevalent freight railcar construction in the United States includes what are known as three-piece trucks. Trucks are wheeled structures that ride on tracks and two such trucks are normally used beneath each railcar body, one truck at each end. The "three-piece" terminology refers to a truck which has two sideframes that are positioned parallel to the wheels and the rails, and to a single bolster which transversely spans the distance between the sideframes. The weight of the railcar is generally carried by a center plate connected at the midpoint of each of the bolsters.
Each cast steel sideframe is usually a single casting comprised of an elongated lower tension member interconnected to an elongated top compression member which has pedestal jaws on each end. The jaws are adapted to receive the wheel axles which extend transversely between the spaced sideframes. Usually, a pair of longitudinally spaced internal support columns vertically connects the top and bottom members together to form a bolster opening which receives the truck bolster. The bolster is typically constructed as single cast steel section and each end of the bolster extends into each of the sideframe bolster openings. Each end of the bolster is then supported by a spring group that rests on a horizontal extension plate projecting from the bottom tension member.
Railcar trucks must operate in severe environments where the static loading can be magnified, therefore, they must be structurally strong enough to support the car and the car payload, as well as the weight of its own structure. The trucks themselves are heavy structural components which contribute to a substantial part of the total tare weight placed upon the rails. Since the rails are typically regulated by the railroads, who are concerned with the reliability and the wear conditions of their tracks, the maximum quantity of product that a shipper may place within a railcar will be directly affected by the weight of the car body, including the trucks themselves. Hence, any weight reduction that may be made in the truck components will be available for increasing the carrying capacity of the car.
The designers of the early cast steel trucks experimented with several types of cross sections in their quest to reduce sideframe weight, but were unable to develop a successful "open" cross section. In fact, the efforts were so unsuccessful that, to this day, the Association of American Railroads (AAR) prohibits open section sideframes. Modern cast steel sideframes currently used in the three-piece truck configurations are designed with cross sections having either a box or C-shape. To produce these cross sections, numerous cores must be used in the molding process, but the use of cores increases production costs and complicates the pouring process by adding complex channels inside the mold which must be filled with molten metal.
Fabricated sideframes were later experimented with, and they were seen as a revolutionary light weight replacement for the cast sideframe. However, the presence of welds in the fabricated sideframes were found to reduce fatigue life and hence, structural integrity of the sideframe, as compared to the cast structures. As a result of the low service life for fabricated sideframes, interest in the cast steel sideframes continued, but in order to improve the fatigue life, it became necessary to increase the structural cross-sectional thicknesses, which is a negative focus for obvious reasons.
Another problem hindering the development of lighter, yet stronger sideframes was the fact that structural development of a cast steel sideframe design is extremely expensive and prior to the modern computer, the load paths on a sideframe could only be valuated after producing an expensive pattern and then pouring a test sample piece. Typically, the manufacturing process required several samples to be cast in order to produce a single part acceptable for testing. Furthermore, the loading tests which predict sideframe structural integrity are expensive and only a few machines exist which are officially approved by the AAR for verification purposes; one of those being at the ASF lab in Granite City, Ill. Nevertheless, even after all of the developmental stages have been completed, the AAR must still approve the design change. This process can take months, even years, for a complex design change. Therefore, it is not surprising that innovation in the railroad industry has proceeded slowly in the freight car truck design area. In spite of these handicaps, new analytical tools and a genuine need to help the railroads reduce costs is now at hand.
However, with the great strides made in development of computer technology, advanced engineering analysis has allowed designers to challenge these principles and to design car members which are actually stronger, yet lighter, than past designs. These latest techniques have increased the focus of attention towards maximizing the carrying capacity of the car while reducing the energy consumption realized from weight reductions in the railcar components.